Pascal Morfin

Dissipating quartets of excitations in a superconducting circuit

  1. Aron Vanselow,
  2. Brieuc Beauseigneur,
  3. Louis Lattier,
  4. Marius Villiers,
  5. Anne Denis,
  6. Pascal Morfin,
  7. Zaki Leghtas,
  8. and Philippe Campagne-Ibarcq
Over the past decade, autonomous stabilization of bosonic qubits has emerged as a promising approach for hardware-efficient protection of quantum information. However, applying these
techniques to more complex encodings than the Schrödinger cat code requires exquisite control of high-order wave mixing processes. The challenge is to enable specific multiphotonic dissipation channels while avoiding unintended non-linear interactions. In this work, we leverage a genuine six-wave mixing process enabled by a near Kerr-free Josephson element to enforce dissipation of quartets of excitations in a high-impedance superconducting resonator. Owing to residual non-linearities stemming from stray inductances in our circuit, this dissipation channel is only effective when the resonator holds a specific number of photons. Applying it to the fourth excited state of the resonator, we show an order of magnitude enhancement of the state decay rate while only marginally impacting the relaxation and coherence of lower energy states. Given that stray inductances could be strongly reduced through simple modifications in circuit design and that our methods can be adapted to activate even higher-order dissipation channels, these results pave the way toward the dynamical stabilization of four-component Schrödinger cat qubits and even more complex bosonic qubits.
arxiv pdf

Stabilizing the trajectory of a superconducting qubit by projective measurement feedback

  1. Philippe Campagne-Ibarcq,
  2. Emmanuel Flurin,
  3. Nicolas Roch,
  4. David Darson,
  5. Pascal Morfin,
  6. Mazyar Mirrahimi,
  7. Michel H. Devoret,
  8. Francois Mallet,
  9. and Benjamin Huard
Making a system state follow a prescribed trajectory despite fluctuations and errors commonly consists in monitoring an observable (temperature, blood-glucose level…) and reacting
on its controllers (heater power, insulin amount …). In the quantum domain, there is a change of paradigm in feedback since measurements modify the state of the system, most dramatically when the trajectory goes through superpositions of measurement eigenstates. Here, we demonstrate the stabilization of an arbitrary trajectory of a superconducting qubit by measurement based feedback. The protocol benefits from the long coherence time ($T_2>10 mu$s) of the 3D transmon qubit, the high efficiency (82%) of the phase preserving Josephson amplifier, and fast electronics ensuring less than 500 ns delay. At discrete time intervals, the state of the qubit is measured and corrected in case an error is detected. For Rabi oscillations, where the discrete measurements occur when the qubit is supposed to be in the measurement pointer states, we demonstrate an average fidelity of 85% to the targeted trajectory. For Ramsey oscillations, which does not go through pointer states, the average fidelity reaches 75%. Incidentally, we demonstrate a fast reset protocol allowing to cool a 3D transmon qubit down to 0.6% in the excited state.
arxiv pdf